The present invention concerns films, sheets, foils and similar flexible, transparent composites consisting of several coextruded layers having high inter-layer compatibility and adhesion, suitable, in particular for the manufacture, by forming-filling-sealing processes (F-F-S), of containers. Such containers include bags of fluids, particularly of infusional solutions which must be sterilized in an autoclave with steam at about 121xc2x0 C. or more, and must have, besides transparency and good welding resistance, high resistance to puncture and fall. The films generally comprise at least an outer layer (A), and inner layer (C), and a complex intermediate layer (B) between (A) and (C). The outer layer (A) goes into contact with the soldering means during said F-F-S process and has a melting temperature of at least 130xc2x0 C. The outer layer (A) also has a high mechanical strength to sustain the filled up container during the sterilization phase. The inner layer (C) is destined to go in contact with said fluids and, once the container is formed and filled up, is responsible of the soldering hot or cold seals. The complex intermediate layer (B) between (A) and (C) is particularly important for the adhesion, transparency, mechanical resistance, and shock absorption of the film. Intermediate layer (B) comprises substantially olefin polymers in absence of adhesives and crosslinking compounds.
In patent application (Swiss Pat. Application No. 3771/93, corresponding to EP-A-0658421) is described a multilayer coextruded film substantially of the above type, i.e. having the characteristics indicated as prior Art in the preamble of the present specification and relevant claim 1. In said Application (the description of which is intended to be herein incorporated), the outer layer (A) and the inner layer (C) had the same symmetrical composition (i.e. the same sterilizable polyolefin PO-STERI) whereas the intermediate layer (B), (not sterilizable, with melting point below 121xc2x0 C. and thickness from 50 to 200 microns) was constituted by polyolefins selected from the group consisting of thermoplastic polyolefins, ethylene-butene copolymers with density below 0.9 g/cm3 and relevant blends. The materials of layers (A and C) (having a thickness from 10 to 80 microns) were selected among: polymers and copolymers of propylene with ethylene and/or butylene; polymers or copolymers of ethylene with an alpha-olefin having six carbon atoms; relevant blends with or without minor amounts of elastomers. The following symbols will be used to indicate: PP, the propylene polymers and PE the ethylene polymers which can be linear (L) and have low (L), medium (M) or high (H) density (D).
In the examples layers (A) and (C) were both and contemporaneously PP or C8-LLDPE (linear, low density polyethylene with small amount of octene, (8)) whereas (B) could be also polyolefinic recovered and regranulated material. Similarly in EP-A-0216506 (classified X i.e. considered relevant in the Search Report of said EP-A-0658421) medical bags are described which are formed by laminates consisting of layers (A) and (C) both equal and selected among LDPE, HDPE or ethylen-alpha olefin copolymers with a density of at least 0.920 g/cm3 whereas (B) is an ethylen-alpha olefin copolymer having density below 0.935 g/cm3. The necessity of having layers (A) and (C) of symmetrical composition was, in general, attributable to the fact that, in so doing it reduced the difficulties of the intermediate layer (B) to confer adequate compatibility and adhesions between said layers (A) and (C) which just because they had a same coposition were already compatible.
The experience has, however, demonstrated that it is neither easy nor sure to confer with a sole layer (even if very thick) of the material of (B) adequate adhesion, particularly after sterilization, to layers (A) and (C) not withstanding their equal composition. Indeed in the case of infusional solution pouches the problems to face are two-fold and concern not only the evaluation and selection of the (A) and (C) couples more suitable, for instance, in terms of viscosity, but also the problems in terms (f.i.) of the acceptability by the xe2x80x9cPHARMACOPEAxe2x80x9d for which interlayer adhesion values are to be respected which must be very high and critical, indispensable to guarantee an acceptable behaviour of the structure. There is moreover the current request of bags with always increasing solution volume, which must therefore show resistance characteristics adequately higher.
The main object of the present invention is that of providing coextruded films and relevant containers having a xe2x80x9cmaximum maximorumxe2x80x9d of characteristics, in particular an optimal combination of excellent values, including the adhesion between terminal layers (A) and (C) of different composition the compatibility, transparency, softness, fall resistance, seal strength, and the absence of transfer of decomposition products in the solutions etc.
This and other objects are reached with the films and related containers having the features recited in the claims. It has indeed been found that by advantageously using layers (A) and (C) of different composition and, between them, a discreet series of substrates B1, B2 . . . Bn critically distributed with gradients as well of their content of combined monomeric ethylene (E) units, as of their melting (softening) temperature, it is possible to obtain films and bags able to satisfy even the most impellent exigencies, including: resistance, machinability, and presentation, etc., in addition to the Pharmacopea specific requirements.